1. Field of the Invention
The present invention relates to lithium secondary batteries, negative electrodes for the lithium secondary batteries, and methods of manufacturing negative electrodes. More particularly, the invention relates to a lithium secondary battery that employs a negative electrode comprising a negative electrode current collector and a negative electrode mixture layer formed on the negative electrode current collector, the negative electrode mixture layer containing a binder and negative electrode active material particles of silicon and/or a silicon alloy, and specifically to an improvement in the negative electrode for the lithium secondary battery so that charge-discharge cycle performance can be improved.
2. Description of Related Art
In recent years, a lithium secondary battery using a non-aqueous electrolyte and which performs charge-discharge operations by transferring lithium ions between its positive and negative electrodes has been in use as a new type of high power, high energy density secondary battery.
One type of negative electrode that is used for such a lithium secondary battery is a negative electrode that has a negative electrode current collector and a negative electrode mixture layer formed on a surface of the negative electrode current collector, the negative electrode mixture layer containing a binder and a material capable of alloying with lithium.
In a lithium secondary battery that has a negative electrode using a material capable of alloying with lithium as its active material, however, the negative electrode active material expands and shrinks in volume during the occlusion and release of lithium as the lithium secondary battery undergoes charge-discharge cycles, causing the negative electrode active material to pulverize or peel off from the current collector. This leads to the problem of degradation in current collection performance of the negative electrode, which consequently results in degradation in charge-discharge cycle performance of the lithium secondary battery. The expansion and shrinkage of the negative electrode active material in volume is exacerbated especially when silicon and/or a silicon alloy, which show excellent capability of occluding and releasing lithium, is/are used as the material which alloys with lithium for the purpose of enhancing the capacity of the lithium secondary battery. Consequently, the use of silicon and/or a silicon alloy tends to worsen the problems of degradation in battery capacity during early stages of charge-discharge cycle operations, and of degradation in the charge-discharge cycle performance of the battery.
In recent years, a negative electrode for a lithium secondary battery has been suggested that can improve charge-discharge cycle performance of a lithium secondary battery. The negative electrode comprises a negative electrode current collector made of a conductive metal foil, and a negative electrode mixture layer formed on a surface of the negative electrode current collector that has been roughed so that the surface roughness Ra is 0.2 μm or greater. The negative electrode mixture layer is composed of a binder and negative electrode active material particles of silicon and/or a silicon alloy, and the negative electrode mixture layer is formed by sintering under a non-oxidizing atmosphere. This configuration enhances adhesion between the negative electrode mixture layer and the negative electrode current collector, and thus improves charge-discharge cycle performance of the lithium secondary battery. (See, for example, Japanese Published Unexamined Patent Application No. 2002-260637.)
However, even with the just-described negative electrode for a lithium secondary battery, adhesion between the negative electrode mixture layer and the negative electrode current collector is not necessarily sufficient. Moreover, when the surface roughness of the negative electrode current collector is increased for the purpose of enhancing the adhesion between the negative electrode mixture layer and the negative electrode current collector, the thickness of the negative electrode current collector accordingly increases, resulting in the problem of degradation in the capacity per unit volume of the lithium secondary battery.